This invention relates to a method and apparatus for detecting cracks in bearings, and more particularly to a method and apparatus for detecting microcracks in a bearing having rolling elements, during operation.
In general, if strain energy has been accumulated within a component part of a bearing, a phenomenon, such as plastic deformation, transformation, or breakage, may occur within the component part, when load is applied thereon, to release the strain engergy. At the same time, a microcrack may be developed at a portion of the component part where the phenomenon occurs. If this phenomenon continues to occur, the microcrack grows and finally brings about breakdown of the component part.
In the meanwhile, so-called acoustic emission has also long been known which is a phenomenon that the shock of release of the strain energy is propagated through the component part as an elastic wave.
Conventionally, devices for foreknowing the breakdown of bearings have been proposed, e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 53-43588 and 63-271132, Japanese Provisional Utility Model Publication (Kokai) No. 60-172056, and U.S. Pat. No. 4,768,380. In such a device, a sensor detects acoustic emission (hereinafter referred to as "AE") due to occurrence of a microcrack in a component part of a slide bearing or a rolling bearing. The output, i.e. AE signal pulses, from the sensor is filtered by a filter to remove therefrom noise resulting from mechanical vibrations etc. The amplitude of the noise-removed AE signal pulses is compared with a predetermined reference value voltage), or AE signal pulses are counted by a counter and the frequency, i.e. the number of counted AE signal pulses per unit time is compared with a predetermined reference value, to thereby foreknow the breakdown of the bearing.
However, in the conventional device, since noise having the same frequency band as the AE signal cannot be removed even by the filter, the predetermined reference voltage value for determination of the amplitude of the AE signal must be set to a value considerably higher than the noise level in order to avoid erroneous determination of the AE signal Therefore, the device cannot detect a microcrack unless a considerably great AE is produced, which makes early detection of the microcrack impossible.
Further, in the case of counting the AE signal pulses, there is a problem that the counted value is affected by noise in the AE signal, which makes accurate detection of the microcrack impossible.